Mitt Romney changed the game with his aggressive, confident performance in Wednesday’s Denver debate, erasing the specter of doom that dogged his campaign for weeks.

President Barack Obama’s forces had hinted that all they needed was one good punch to knock out Romney after the Republican spent the summer and early fall stumbling. Instead, the Romney whom viewers saw Wednesday was the one his friends have long known: the conversational, smart, decent-on-his-feet guy, eager to defend his plans to cut taxes and change government health insurance for future generations.

An even cursory dig into the details of those Romney plans – details that are often elusive – will continue to give Democrats ammunition against him. Obama also could recover quickly by relentlessly pressing his opponent on the details. Democrats tried hard Thursday to make those points.

Obama also can take solace in history, which shows that incumbent presidents often falter in first debates – see George W. Bush in 2004 or Ronald Reagan in 1984 – and then come back and win.

But a strong performance by a challenger does reshape the race in several ways, ways that were instantly evident Thursday. Romney made an unscheduled appearance at a conservative conference in Denver, where he got a rousing, standing ovation. People in the crowd vowed that they’d eagerly work for a nominee whose conservative credentials they’d questioned only weeks ago.

That kind of reaction means that Romney donors and grass-roots workers now have a badly needed shot of momentum and energy.

Wavering voters also pay fresh attention. The undecideds tend to tune in to politics only rarely but they’ll notice at least part of a debate. Such voters are usually younger, without the kinds of steady political allegiances that ultimately drive many voters to reluctantly back their parties’ candidates.

Another group of voters to watch is the persuadables, folks who say they’ll vote one way but still can be swayed. They’re the ones who say they’ll go with Obama because they don’t like Romney or vice versa.

A bravura debate performance can change that equation. Suddenly, they may have someone to vote for, not against.

A debate such as Wednesday’s also spurs other changes. The culture takes notice. A Pew Research Center poll recently found that people thought Clint Eastwood’s encounter with an empty chair at the Republican National Convention was the event’s highlight, not Romney’s acceptance speech.

In the coming days, should Obama’s performance get lampooned on “The Daily Show,” “Saturday Night Live,” Leno and Letterman, it could create an impression that lasts, an image of the president as too academic, too cold, uneasy with being challenged.

“Obama’s life and his operating style had been built around avoiding confrontation,” Bob Woodward wrote in his book “The Price of Politics,” as he described the president’s frosty relationship with Democratic congressional leaders.

That style was apparent Wednesday. Toward the end of the debate, Romney, known as a friendly, easy conversationalist, offered a rambling but upbeat assessment of how he’d govern.

“As president I will sit on day one – actually the day after I get elected – I’ll sit down with leaders, the Democratic leaders as well as the Republican leaders, and continue, as we did in my state, we met every Monday for a couple of hours, talked about the issues and the challenges in the – in the – in our state in that case,” he said.

Obama gave a dismissive look and quipped that Romney is “going to have a busy first day, because he’s also going to repeal Obamacare, which will not be very popular among Democrats as you’re sitting down with them.”

The Obama camp was clearly rattled after Wednesday’s debate.

“Gov. Romney came to give a performance. He gave a good performance, and we give him credit for that,” Obama campaign senior adviser David Axelrod said Thursday.

He defended the president’s decision not to raise the incendiary issues he’s used in his TV ads against Romney, saying they didn’t come up in the questions.

“He made a choice last night to answer the questions that were asked,” Axelrod said, “and to talk to the American people about what we need to do to move forward, not get into serial fact-checking with Gov. Romney, which can be an exhausting, never-ending pursuit.”

But any such logic Thursday was overwhelmed by the grim mood. After the debate, twice as many Romney surrogates appeared to talk to reporters. The Obama people left the scene quickly.

Two out of three people surveyed thought Romney did a better job, according to a CNN/ORC International poll. CBS polled uncommitted voters, and they gave Romney a 46-22 percent edge.

None of this means that Romney is about to surge. Democratic nominee Walter Mondale thought he had momentum after the first 1984 debate, when Reagan appeared vague and confused, raising questions about whether, at 73, he could still do the job. But Reagan recovered in the next debate two weeks later and went on to win 49 states.

Romney still has to show that he can connect with ordinary voters. Independent political analyst Charles Cook, who’d been critical of the Romney campaign, said Thursday that that connection “partially happened last night.”

But on Thursday, it was Obama who was on the defensive.

“This was the first time the president really had to answer for his record,” Romney senior strategist Stuart Stevens said.